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1 Early Human Evolution Overview and Chronology What makes us human? Ardipithecus and early Australopithecus Robust and gracile australopithecines Oldowan tools Overview First hominins appeared late in the Miocene, but most hominin fossils date to Pilocene and Pleistocene epochs Hominid Family that includes apes and humans (both current and fossil) Hominin Tribe that includes humans and early ancestors Gibbons Orangutans Gorillas Just orangutans in family Pongidae Chimps Humans Family Hominidae inclusive of humans, chimps, and gorillas Chronology of Hominid Evolution Epochs of the Cenozoic Era If we compare Earth's history to a 24-hour day (1 second = 50k yrs) Earliest fossils deposited at 5:45 a.m. First vertebrates appeared at 9:02 p.m. Earliest mammals, at 10:45 p.m. Earliest primates, at 11:43 p.m. Earliest hominins, at 11:57 p.m. Homo sapiens arrived 36 seconds before midnight Hominins emerged in late Miocene, but most fossils are from Pliocene and Pleistocene epochs. Overview of Species Table 8.1 Dates and Geographic Distribution of Major Hominoid, Hominid, and Hominin Fossil Groups Ardipithecus (6 million years) Australopithecus (4 million years) A. Africanus and A. Robustus By 2 million years ago two distinct hominin groups Homo and A. boisei Early hominin sites 1

2 Chronology of Hominin Evolution Most important epochs for study of hominin evolution Pliocene (5-2mya) Pleistocene (2mya-10kya) Recent (10kya-present) Australopithecus was main hominin genus until end of Pliocene Genus Homo evolved from Australopithecus by start of Pleistocene Figure 8.1 Phylogenetic Tree for African Apes, Hominids, and Hominins The presumed divergence date for ancestral chimps and hominins was between 6 and 8 m.y.a. Branching in later hominin evolution is also shown. For more exact dates, see the text and Table 8.1. What Makes Us Human? A number of traits differentiate hominins from their ancestors Bipedalism Trend toward large brain size Longer period of childhood dependency Trend towards complexity in tool use Trend toward smaller back teeth Bipedalism Hominins have bipedal locomotion they walk on two legs A number of adaptive scenarios have been developed to account for the evolution of bipedalism: Sentinel hypothesis Home base hypothesis Solar radiation hypothesis Efficient gait hypothesis East Side Story Adaptive story by Yves Coppens that accounts for: Divergence of hominins from apes Evolution of bipedalism Late Miocene saw climate change and shrinking of forest habitat; common ancestor split in two populations: One population stayed in forest led to modern chimps Second population moved out into the savannah grassland led to bipedal hominins Great Rift Valley of Africa Figure 8.3 Comparison of Homo sapiens and Pan troglodytes (the Common Chimp). (a) Skeleton of chimpanzee in bipedal position; (b) skeleton of modern human; (c) chimpanzee and human bisected and drawn to the same trunk length for comparison of limb proportions. The contrast in leg length is largely responsible for the proportional difference between humans and apes. 2

3 Figure 8.4 A Comparison of Human and Chimpanzee Pelvises The human pelvis has been modified to meet the demands of upright bipedalism. The blades, (ilia, singular, ilium) of the human pelvis are shorter and broader than those of the ape. The sacrum, which anchors the side bones, is wider. The australopithene pelvis is far more similar to that of Homo than to that of the chimpanzee, as we would expect in an upright biped. Brain Size Earliest hominins had brains not much bigger than modern apes Later hominins i had progressively larger brains Early hominins Primitive brains Derived postcrania Childhood Dependency Hominins have longer period of childhood dependency for body and dba brain go growth May have been a byproduct of evolution of bipedalism Bipedal pelvis requires baby with small head Tool Use Capacity for tool use and culture is a primitive trait shared by humans and some other hominoids Trend toward more complex tools in hominins Earliest stone tools 2.5 mya Dental Changes Figure 8.5 A Comparison of the Skull and Dentition (Upper Jaw) of Homo and the Chimpanzees Early hominins have primitive large back teeth and thick enamel Later hominins do not Canines reduced d from hominoid ancestors One later hominin had extremely large teeth, but this was a unique adaptation not shared hominins ancestral to modern humans 3

4 Ardipithecus Lived during late Miocene, between 5.8 and 5.5 million years ago Eventually evolved into australopithecines i Distinction between australopithecines and later hominins made on genus level Kadabba finds consisted of 11 specimens that were apelike in size, anatomy, and habitat Kenyanthropus Complicating picture is discovery, which Maeve Leakey named Kenyanthropus playtops Shows at least two hominin lineages existed as far back as 3.5 million years ago Kenyanthropus has flattened face and small molars Lucy may not be a direct human ancestor Australopithecus Species A. anamensis (4.2 to 3.9 m.y.a.) A. afarensis (3.8? to 3.0 m.y.a.) A. africanus (3.0? to 2.0? m.y.a.) A. garhi (2.5 m.y.a) A. robustus (2.0? to 1.0? m.y.a.) A. boisei (2.6? to 1.2 m.y.a.) Discovery of Taung Baby First australopithecine discovered by Raymond Dart in 1925 Specimen was a juvenile referred to as the Taung Baby Developed the Killer Ape theory Found osteodontokeratic tools Claimed they were for early warfare 4

5 Australopithecus anamensis Fossils reported first by Leakey and Walker date to m.y.a. Molars have thick enamel and apelike canines are large Weighed about 110 pounds (50 kg) Bipedal May be ancestral to A. afarensis Anamensis was an apelike hominin Australopithecus afarensis A. afarensis and the Laetoli Footprints A. afarensis lived between 3.8 and 3.0 m.y.a. Similar in many ways to chimps and gorillas Indicates common ancestry with African apes must be recent Very small brain case Below neck, unquestionably human Striding bipedalism Table 8.2 Facts about the Australopithecines Compared with Chimps and Homo 5

6 Gracile & Robust Australopithecines I Two groups of South African australopithecines (3 1 m.y.a.) Gracile smaller and lighter Some argue graciles lived before robust (3 2 m.y.a.) Others contend graciles and robusts overlapped Others view them as opposite ends of a continuum Gracile & Robust Australopithecines II Gracile & Robust Australopithecines III Trend toward enlarged back teeth, chewing muscles, and facial buttressing, already noticeable in A. afarensis, continues in South African australopithecines Might have hunted small and slow-moving game Diet mainly vegetarian Teeth of robust Australopithecine Contrasts with Homo in that front teeth are less marked In Robust australopithecines, chewing muscles strong enough to produce sagittal crest Brain size increased only slightly between A. afarensis (430 cm 3 ), A. Africanus (490 cm 3 ), and A. robustus (540 cm 3 ) Robusts probably did not use tools to large extent Early Homo Figure 8.6 Skulls of Robust (left) and Gracile (Right) Australopithecines Showing Chewing Muscles Flaring cheek arches and, in some robusts, a sagittal crest supported this massive musculature. The early hominin diet coarse, gritty vegetation of the Savanna demanded such structures. These features were most pronounced in A. boisei. The Australopithecines and Early Homo Ancestors of Homo split off and became reproductively isolated from later australopithecines between 3 and 2 m.y.a. Homo erectus had larger brain and reproportioned skull H erectus hunted and gathered, made sophisticated tools, and eventually displaced its sole surviving cousin species, A. boisei Johanson and White propose that A. afarensis effectively produced two populations 6

7 Australopithecines and Early Homo 1985 discovery of the black skull (dated 2.6 m.y.a.), apparently an early A. robustus, made for more possible models of the divergence between Homo and Australopithecus Surprising mixture of australopithecine features, particularly given its relatively early date The Black Skull Oldowan Tools Oldest tools from Olduvai are widely used between 1.5 and 2 m.y.a. Stone tools consist of cores and flakes Core-piece of rock from which flakes are removed Chopper-tool made by flaking the edge of such a core on one side Oldowan pebble tools represent world s oldest formally recognized stone tools A. Garhi and Early Stone Tools In 1999, a new hominid species, A. garhi, found in Ethiopia associated with stone tools and the remains of butchered animals Added new species to human family tree Demonstrated the thigh bone elongated one million years before the forearm shortened to create current human proportions Showed early stone tools designed at getting meat and marrow from big game 7

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